fec: use interrupt for MDIO completion indication

With the move to phylib (commit e6b043d) I was seeing sporadic "MDIO write
timeout" messages. Measure of the actual time spent showed latency times of
more than 1600us.

This patch uses the MII event indication of the FEC hardware to detect
completion of MDIO transactions.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>